The present invention relates to cellular radio telecommunication systems. More particularly, and not by way of limitation, the present invention is directed to a method and a base station controller (BSC) for handing over a mobile station conducting both a circuit-switched call and a packet data session, and rapidly reestablishing the packet data session.
The GSM/EDGE system is the most widely used cellular standard in the world. This system provides access to mobile telephony services using a core network that can support both circuit switched (CS) and packet switched (PS) services. While CS services are related to mobile telephony, PS services primarily deal with Internet connectivity. Details regarding the provision of CS and PS services in the GSM/EDGE system are provided in the technical specification 3GPP TS 23.060 v6.7.0 entitled, “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; General Packet Radio Service (GPRS); Service description; Stage 2 (Release 6)”, December 2004, which is fully incorporated by reference herein.
As described in the GSM/EDGE specifications, a mobile station (MS) that participates in a circuit call is said to be in “dedicated mode” of operation. For purposes of this disclosure, an MS that is not in dedicated mode will be referred to as being in “idle mode”. Likewise, an MS engaged in active packet transfer over GPRS is said to be in “packet transfer mode” or “GPRS active mode”. All other MSs are in the “packet idle mode”.
Within the GSM/EDGE system, an MS may be classified in one or more classes: Class A, Class B, or Class C. Class-A MSs may attach to both GPRS and other GSM services, and support simultaneous attach, activation, monitor, invocation, and traffic operations. The mobile user can make and/or receive calls on the two services simultaneously, subject to Quality of Service (QoS) requirements. A minimum of one channel shall be available for each type of service (CS and GPRS) when required.
Class-B MSs may attach to both GPRS and other GSM services, but the MS can only operate one set of services at a time. Thus, the MS must terminate an ongoing PS transaction in order to participate in a CS voice call. Class-C MSs may attach to either GPRS or other GSM services, but not both simultaneously.
In general, the GSM/EDGE network independently allocates radio resources to the CS and PS domains. This is because the Radio Resource (RR) management entity, hereafter referred to as RR, does not have an associated context for an MS unless the MS is in “dedicated mode”, meaning there is either a call in progress in the circuit network or there is ongoing packet communication between the MS and the network. Moreover, the RR does not coordinate the allocation of radio resources between the CS and PS domains. Therefore, the Class-A MS must, strictly speaking, have two independent radios, since the CS allocation could be on a different carrier frequency than the PS allocation. Even if the carriers are identical, there is no guarantee that the CS and PS domains would honor the multi-slot class of the MS, which determines the combinations of timeslots on which the mobile station is capable of transmitting and receiving. Obviously, this strict interpretation of the specification causes significant problems in building MSs that allow simultaneous circuit and packet domain operation.
These problems have been addressed in the past through the introduction of a new Class-A mode known as simple Class-A or Dual Transfer Mode (DTM) and described in concept within the 3GPP technical specification ETSI TS 102 355 v8.4.0 entitled, “Digital Cellular Telecommunications system (Phase 2+); Dual Transfer Mode (DTM); Stage 2 (3GPP TS 03.55 version 8.4.0 Release 1999)”, January 2005, which is fully incorporated by reference herein. DTM allows for coordination between the radio resource allocation in the CS and PS domains. Moreover, DTM allows PS domain control messages to be sent on the main Digital Control Channel (DCCH). The main DCCH for an MS during a call is the Fast Associated Control Channel (FACCH). When these PS domain control messages are sent from an MS to a Serving GPRS Service Node (SGSN), the messages are tunneled to the Base Station Subsystem (BSS), and the BSS unpacks packet domain information from the FACCH messages and forwards the corresponding PS messages to the SGSN. However, this process generally incurs a lengthy delay.
Another function related to the present invention is handover. In existing handover procedures for change of cell or location area, the CS domain takes precedence over the PS domain. In such circumstances, the packet session is suspended while the MS performs a normal handover. Packet communication is then reestablished via a cell update after the MS establishes CS communication in the new cell. This entire process may cause delays of several seconds for the PS communication, thus causing a significant loss of quality-of-service (QoS) during the process. While the GPRS network was originally designed primarily as a best-effort network, the deployment of EDGE and EGPRS, and the availability of higher data rates on competitive networks such as UMTS and CDMA2000, has made QoS more important within GSM/EDGE. In one particular application, for example, there is a need for the voice and image information being transferred simultaneously during DTM to be loosely coordinated during a handover. In the current network, the break of packet data communication of several seconds is excessively long, and interferes with user satisfaction.
FIG. 1 is a signaling diagram illustrating the flow of messages for an internal, dual mode handover in accordance with existing 3GPP standards. A mobile station (MS) 11 is communicating through a GSM/EDGE network. The network is shown to include an old (serving) base transceiver station (BTS) 12, a new (target) BTS 13, a base station controller (BSC) 14, a mobile switching center (MSC) 15, and a Serving GPRS Service Node (SGSN) 16. The MS is shown to be a dual-mode MS conducting a CS call 17 through the MSC and a PS session 18 through the SGSN.
As the MS 11 moves away from the old BTS 12 into the cell served by the new BTS 13, the BSC 14 serving the two cells may activate channels in the target cell by sending a Channel Activation message 19 to the new BTS and receiving a Channel Activation Acknowledgment message 21 in response. The BSC may then send a Handover Command message 22 requesting the MS to switch to the new BTS. At the receipt of the Handover Command message, the MS abandons the PS session 18 and initiates handover access 23 to the new BTS, obeying the handover time requirements of 3G TS 05.10 [8] clause 6 and 3G TS 04.13 [3] clause 5.2.6.
At 24, the CS connection is reestablished as a CS-only handover, and when concluded, the BSC 14 sends an RF Channel Release message 25 to the old BTS 12. The old BTS sends an acknowledgement 26 and releases the channels in the old cell. The CS call 17 then continues. If the MS 11 was in dual transfer mode in the old cell, the BSC sends a DTM Information message 27 to the MS, with information needed to resume the GPRS operation. Once the MS has the necessary information, it performs a Cell Update or RA Update procedure 28. The Cell Update procedure may be performed on the main DCCH or on a temporary block flow (TBF), as shown in FIGS. 2 and 3. If the MS also needs to (re-)establish an uplink PS session in the new cell, the GMM signaling procedure takes precedence and is performed first. Once the update procedure is performed, the PS session 18 is (re-)established.
In the case of an external, dual mode handover, the new (target) base station subsystem (BSS) is provided with the IMSI of the MS 11 and with information about the nature of the packet resources in the old (serving) cell, so that the CS resource is compatible with the PS resources that are going to be requested in the new cell (for example, a transceiver supporting AMR or EDGE, a timeslot with a free, adjacent timeslot, and the like). This information is conveyed in an Old-BSS-to-New-BSS Information Element (IE). Since this IE is optional, if the target BSS does not have any knowledge of the RR mode of the MS, it sends the DTM Information message.
The same essential procedures are envisaged for an inter-MSC handover. Current implementations are expected to be able to carry the extended Old-BSS-to-New-BSS IE without modifications to 3G TS 09.08. Likewise, no changes are foreseen for an inter-SGSN handover. The MS performs a Routing Area Update procedure in the new cell.
FIG. 2 is a signaling diagram illustrating the flow of messages involved in a Cell Update procedure performed on the main DCCH when the MS 11 is in CS dedicated mode, packet idle mode, and Ready state according to existing 3GPP standards. In this scenario, the MS has requested uplink resources, indicating “Cell Update”, and the BSS 31 has commanded the MS to perform the Cell Update procedure in single timeslot operation. As shown, the MS is in CS dedicated mode 32 with the MSC 15, and sends GPRS Transparent Transport Protocol (GTTP) information 33 to the BSS 31 in a Logical Link Control Packet Data Unit (LLC PDU). The BSS then sends uplink UL-UNITDATA 34 in an LLC PDU to the SGSN 16 to complete the Cell Update procedure. As stated before, the processing involved at the BSS incurs extra delay that translates into a degraded QoS.
FIG. 3 is a signaling diagram illustrating the flow of messages involved in a Cell Update procedure performed on a temporary block flow (TBF) when the MS 11 is in CS dedicated mode, packet idle mode, and Ready state according to existing 3GPP standards. Once again, the MS has requested uplink resources, indicating “Cell Update”. In this case, however, the BSS 31 has allocated an uplink TBF on a different time slot. The MS sends a DTM Request message 35 to the BSS, which returns a Packet Assignment Command 36. The MS then sends one or more Radio Link Control/Medium Access Control (RLC/MAC) blocks 37 to the BSS. The BSS then sends uplink UL-UNITDATA 34 to the SGSN 16 to complete the Cell Update procedure. As previously noted, this entire process may cause delays of several seconds for the PS communication, thus causing a significant loss of QoS during the process.
What is needed in the art is a method and BSC for reestablishing packet data services following a dual-mode handover that overcomes the disadvantages of the prior art. The present invention provides a method and BSC that overcome the disadvantages of the prior art under a select set of scenarios.